Currently, integrated circuit (IC) chip patterns are etched out of semiconductors using photolithography, ultraviolet lithography, or x-ray lithography. Photolithography ultraviolet, and x-ray lithography are processes by which a semiconductor is first covered with a layer of photosensitive material (i.e. photoresist). This layer is then covered with a stencil-like mask in the shape of the desired electric circuit. When the semiconductor is exposed to visible light, ultraviolet light, or x-rays a chemical reaction occurs in the exposed areas of the photosensitive material or photoresist Then, depending on the process, either the exposed or unexposed area of the photosensitive material or photoresist is etched away by acid (i.e. a mixture of H.sub.2 SO.sub.4 and H.sub.2 O.sub.2), leaving a pattern in the shape of the mask.
One drawback with all lithography methods, including x-ray lithography, is that extraordinary care is required to avoid defects in the masks since masks are used to manufacture millions of chips per mask. Because these masks create extremely complex and dense patterns millions of times over, inspection of these masks for defects is critical. There are two principal types of mask defects: (1) a spot, which is an area that absorbs radiation where it should not, and (2) a hole, which is an area that does not absorb radiation where it should. These defects may be formed in unpatterned areas, patterned areas, or on feature edges. There can also be hybrids or combinations of these two principal types of defects.
There have been a variety of approaches to x-ray mask inspection. A typical approach involves inspection of the mask with an electron beam in reflection or transmission. The response of a mask to electrons is not identical to its response to x-rays, however, and the rate of inspection is typically slow.
Another approach is exemplified in U.S. Pat. No. 4,718,767 to Hazaam. A copy of a mask is made on a transparent wafer coated with a photosensitive layer, and after development the resulting pattern on the wafer will be optically inspected. Drawbacks with this method are that (1) the wafers must be developed (i.e. chemical processing of the photoresist material exposed to the x-ray source) in order to examine the "spots;" and (2) numerous wafers must be used for multiple inspections throughout a production, which can be relatively expensive. Direct optical inspection is lengthy, tedious, and inaccurate.
Another approach is described in U.S. Pat. No. 5,123,743 to Feldman. Feldman creates two exposures on the same wafer, one through a first mask onto a positive resist on the wafer, and one through a second, ostensibly identical mask, onto a negative resist on the same wafer. Both masks are either positive or negative. The resulting image on the wafer will be either a completely dark or completely light background, with defects represented as islands or "specks" on the wafer. Drawbacks with this method are that (1) the two mask images (positive and negative) must be precisely aligned on the wafer--if not, the necessary dark or light background will not be created; (2) the wafers must be developed in order to examine the "spots;" and (3) numerous wafers must be used for multiple inspections throughout a production run.
Thus, there remains a need for a simple, inexpensive and accurate method of inspecting x-ray lithography patterns.